1. Field of the Invention
The present invention relates to a sound reproducing apparatus and a sound reproducing method for reproducing audio signals as sounds.
2. Description of the Related Art
Heretofore, various sound-effect technologies that achieve target sound localization at an appropriate listening position are employed in sound reproduction systems using a plurality of speakers.
For example, two-channel intensity-control stereophonic reproduction systems or multi-channel surround-sound stereophonic reproduction systems are known.
In the former stereophonic reproduction systems, target sound localization is achieved at a certain listening position, that is, at a listening position that forms a regular triangle together with right-side and left-side speakers.
With the stereophonic reproduction systems described above, reproduced sounds reaching the ears of listeners at the positions other than an appropriate listening position are significantly different from reproduced sounds reaching the ears of listeners at the appropriate listening position. That is, target sound localization cannot be achieved at positions other than the appropriate listening position.
The above problem will now be described with reference to FIGS. 5A to 5C.
FIGS. 5A to 5C illustrate the relationship between an arrangement of speakers and binaural response characteristics of listeners at listening positions.
Referring to FIG. 5A, two speakers (left speaker SL and right speaker SR) are arranged at the right and left sides in front of listeners.
Sounds that reach the ears of a listener at a listening position A at equal distances from the two speakers SL and SR are shown in FIG. 5B. Specifically, a left sound L1 from the left speaker SL reaches the left ear of the listener at the listening position A and a right sound R1 from the right speaker SR reaches the right ear of the listener at the listening position A. In this case, the left sound L1 from the left speaker SL reaches the ears of the listener substantially simultaneously with the right sound R1 from the right speaker SR. The listener at the listening position A perceives a sound image as if it were at a sound-image position a near the center of the two speakers SL and SR.
At this time, a right sound R2 from the right speaker SR is detoured to the left ear of the listener at the listening position A, and a left sound L2 from the left speaker SL is detoured to the right ear of the listener. Accordingly, the right sound R2 reaches the left ear of the listener behind the left sound L1, and the left sound L2 reaches the right ear of the listener behind the right sound R1.
In contrast, sounds that reach the ears of a listener at a listening position B on the right side of the listening position A in FIG. 5A are shown in FIG. 5C. Specifically, the left sound L1 from the left speaker SL reaches the left ear of the listener at the listening position B and the right sound R1 from the right speaker SR reaches the right ear of the listener at the listening position B.
Also at this time, the right sound R2 from the right speaker SR reaches the left ear of the listener at the listening position B, and the left sound L2 from the left speaker SL reaches the right ear of the listener.
Since the right sound R2 from the right speaker SR is detoured to the left ear of the listener at the listening position B and the left sound L2 from the left speaker SL is detoured to the right ear of the listener, the right sound R2 reaches the left ear of the listener behind the left sound L1, and the left sound L2 reaches the right ear of the listener behind the right sound R1. However, the sound R2 reaches the left ear of the listener at a time different from the time when the sound L2 reaches the right ear of the listener because the distance from the left speaker SL to the listener at the listening position B is different from the distance from the right speaker SR to the listener at the listening position B.
Since the listening position B is nearer to the right speaker SR than the listening position A, as shown in FIG. 5A, the right sound R1 reaches the right ear of the listener at the listening position B earlier than the left sound L1 that reaches the left ear of the listener at the listening position B.
In this case, for the listener at the listening position B, the sound image is localized at the position of the right speaker SR owing to the precedence effect achieving the sound-image localization closer toward the right sound R1 that first reaches the right ear of the listener at the listening position B. That is, the listener at the listening position B obtains a sound image at a sound-image position b different from a sound-image position a where the listener at the listening position A obtains a sound image. However, since the left sound from the left speaker SL actually differs from the right sound from the right speaker SR in level or the like, the listener at the listening position B perceives the sound-image position as if it were localized closer toward the right speaker SR.
As described above, in the arrangement shown in FIG. 5A, the listener at the listening position A that is at equal distances from the two speakers SL and SR perceives that the sound image is localized at the target position a, whereas the listener at the listening position B different from the listening position A perceives the sound image as if it were localized at the sound-image position b different from the sound-image position a of the listening position A. In other words, with known stereophonic reproduction systems, the listener at a position different from the listening position A cannot perceive that a sound image is localized at the target sound-image position a.
FIGS. 6A to 6C illustrate the relationship between another arrangement of speakers and the binaural response characteristics of listeners at listening positions, as a comparison with the arrangement of speakers shown in FIGS. 5A to 5C.
Referring to FIG. 6A, one speaker SC is arranged at the center in front of listeners. Sounds C1 from the speaker SC reaching the ears of a listener at a listening position A that is located substantially in front of the speaker SC are shown in FIG. 6B. Referring to FIG. 6B, the sounds C1 from the speaker SC substantially simultaneously reach the right and left ears of the listener at the listening position A. In this case, a sound-image position c coincides with the position of the speaker SC.
In contrast, sounds from the speaker SC reaching the ears of a listener at a listening position B on the right side of the listening position A in FIG. 6A are shown in FIG. 6C. Referring to FIG. 6C, the sound C1 from the speaker SC first reaches the left ear of the listener at the listening position B, and the sound C1 reaches the right ear of the listener after it reaches the left ear of the listener. However, the listener at the listening position A obtains the same sound-image position c as the listener at the listening position B. In other words, the sound-image position is invariable even if the listening position of the listener is changed.
As shown in FIGS. 5A to 6C, speakers that output sounds having different wavefront shapes sound different. That is, speakers sound different in accordance with the arrangement of the speakers and the listening positions. This is because different sound-pressure waveforms are transmitted to the eardrums of a listener when the listener hears sounds from the speaker with his auditory organ, such as the head or the auricle.
A method of outputting sounds from a speaker to reproduce a target sound field is disclosed in Japanese Unexamined Patent Application Publication No. 2-114799. This publication discloses a technique in which arranging speakers at intervals that are not more than half of a wavelength of a maximum spatial frequency of a sound wave and reproducing sounds causes the reproduced sounds to be diffused and to interfere with each other in order to form a wavefront close to an original sound field.
This publication also discloses a technique of picking up sounds for reproducing a wavefront close to an original sound field when speakers are arranged at intervals that are not more than half of a wavelength of a maximum spatial frequency of a sound wave and the sounds are reproduced.
Technologies, relating to the present invention, for appending delayed signals for complementing the sound volume without changing the sound-image position perceived by a listener are disclosed in Japanese Unexamined Patent Application Publications No. 9-149500 and No. 10-243500. However, reproduction of wavefront is not discussed in these two publications.
In the reproducing device disclosed in Japanese Unexamined Patent Application Publication No. 2-114799, it is necessary to set an interval between speakers to a value not more than half of a wavelength of a maximum reproduction frequency (maximum spatial frequency) of a sound field to be reproduced.
Accordingly, multiple speakers must be arranged at short intervals in order to reproduce a sound field across a wide band from several tens of hertz to several tens of kilohertz.
For example, in order to reproduce sounds having a frequency of 17 kHz with speakers and to reproduce a sound field, the speakers must be arranged at intervals of 1 cm and, therefore, it is impossible to actually arrange the speakers at such intervals.
Hence, with the reproducing device disclosed in Japanese Unexamined Patent Application Publication No. 2-114799, it is difficult to output sound waves across a wide band from lower frequencies to higher frequencies by using one wavefront and there is a problem in that a sound field cannot be reproduced across a wide band including higher frequencies.